A polarizing plate 20 used by attaching to a liquid-crystal display device or the like, for example, has a configuration shown in FIG. 1. Thus, a polarizing film 10 is composed by attaching preventive films 3 for a polarizer, which form preventive layers, via an adhesive 7 to both surfaces of the polarizer 4 in a sandwiched manner. A preventive film 1 for a polarizing film is attached adhesively via a pressure sensitive adhesive 6 to one side of the polarizing film 10. A phase difference film 5 is attached to the other side of the polarizing film 10, and a releasable preventive film 2 for a polarizing film in which a releasing agent 8 is applied to one surface of the film of the same configuration as the preventive film 1 for a polarizing film is adhesively attached thereto via a pressure sensitive adhesive 6.
A polarizer obtained by causing adsorption of iodine or a dichroic dye on a film composed of a hydrophilic polymer such as polyvinyl alcohol and orienting the film by stretching is used as the polarizer 4. The following requirements are placed on the preventive film 3 for a polarizer, which maintains endurance and mechanical properties of the polarizer 4: the film has to be optically transparent, that is, should not demonstrate double refraction easily, and also has to have heat resistance, high mechanical strength, a smooth surface, and good adhesion to the adhesive 7 and pressure sensitive adhesive 6, and a non-oriented film such as a triacetyl cellulose can be used. In order to obtain a clear color and fine image, the phase difference film 5 is required to demonstrate uniform double refraction over the entire surface and have stable optical properties even under severe environmental conditions of high temperature and high humidity. Uniaxially stretched films of polycarbonates have been used therefor.
The preventive film 1 for a polarizing film and the releasable preventive film 2 for a polarizing film are used with the object of preventing the surface of the polarizing film 10 or phase difference film 5 from contamination or scratching in the process of distributing the polarizing film 10 or mounting on a liquid-crystal display device. Those preventing films are attached adhesively to the polarizing film 10 or phase difference film 5 with the pressure sensitive adhesive 6, and it is desired that they can be easily separated in the process of mounting on a liquid-crystal display device. It is preferred that in the releasable preventive film 2 for a polarizing film, the releasing agent 8 be applied to the surface of adhesive bonding to the pressure sensitive adhesive 6 to facilitate peeling.
Based on the aforementioned necessary properties, cellulose films such as triacetyl cellulose, polyester films, polyacryl films, polycarbonate films, and polyethersulfone films are used as the preventive film 3 for a polarizer. A polarizing film 10 is composed by attaching such preventive film 3 for a polarizer in a sandwiched manner via an adhesive 7 so as to protect both surfaces of the polarizer 4. The polarizer 4, as described hereinabove, is prepared by causing adsorption of iodine or a dichroic dye on a film composed of a hydrophilic polymer such as polyvinyl alcohol and orienting the film by stretching, but because the iodine is evaporated if the film is thermally fixed after stretch orientation, the film is used without conducting the thermal fixing. For this reason, the film is greatly expanded and shrunk under the effect of temperature changes or moisture changes in the environment. For this reason, the preventive films 3 for a polarizer which are attached to the polarizer 4 by sandwiching and constrict the deformation caused by temperature changes or moisture changes have to be attached by using the adhesive 7 and cured by heating at a temperature of 90° C. or less at which iodine is not evaporated. Directly thermally attaching to the substrate film, without using an adhesive, and attaching the preventive film for a polarizer by using a usual adhesive which is cured by heating at a high temperature are impossible. Adhesives that are cured by heating at a low temperature are special and expensive products and require an adhesive application process. Therefore, they are a significant obstacle for reducing the cost of polarizing plates.
It is an object of the present invention to provide a preventive film for a polarizer that has an extremely small double refraction and can be directly thermally attached to a polarizer, and also to provide an inexpensive polarizing plate using such a film.